FIG. 1 illustrates in a simple fashion an I/O (input/output) driver (buffer, transmitter) 102 communicating with receiver 104 via transmission line 106. Transmission line 106 may be the physical layer of a bus, and has a characteristic impedance Z.sub.0.
Suppose driver 102 has transmitted or is transmitting an electromagnetic wave traveling in a direction denoted by arrow 108. If the input impedance of receiver 104 is not equal to Z.sub.0, then a reflected wave will propagate in the direction indicated by arrow 110. If the impedance of driver 102 is not matched to the characteristic impedance Z.sub.0, then another reflected wave will again be generated, but now traveling in the direction of arrow 108. There will be many multiple reflections, with field vector at any point along transmission line 106 equal to the vector sum (superposition) of the transmitted field vector and all reflected field vectors at that point. This superposition of the transmitted wave and the reflected waves may cause signal degradation, such as ringing, which may limit the speed at which digital data is reliably transmitted from driver 102 to receiver 104.
The first reflected wave can be reduced by terminating the receiving end of transmission line 106 with a receiver or stub having an impedance matched to Z.sub.0. However, this may require the use of an off-chip resistor, and furthermore, power may be wasted due to ohmic losses in the resistor.